Temperature indicator using the smectic c phase of a liquid crystal

ABSTRACT

TEMPERATURES MAY SOMETIMES BE QUICKLY AND CONVENIENTLY SENSED AND/OR MAPPED WITH THE USE OF A VARIABLE-TILT COMPOUND OF THE SMECTIC C MESOMORPHIC PHASE. DISCLOSED ARE SEVERAL ORGANIC COMPOUNDS THAT ARE CAPABLE, WITHIN CERTAIN TEMPERATURE RANGES, OF EXHIBITING SUCH A PHASE, SUCH AS TEREPHTHAL-BIS(4-N-BUTYLANILINE) AND OTHER COMPOUNDS RESULTING FROM THE REACTION OF TEREPHTHALDEHYDE WITH AMINES OR AMIDES OF THE FORMULA:   1-R,4-A-BENZENE   WHERE A IS -NH2 OR -CONH2 AND R IS A SATURATED ALKYL OR ALKOXY GROUP CONTAINING 1-12 CARBON ATOMS. THE COMPOUND IS MADE, APPLIED TO A SURFACE THAT HAS BEEN RUBBED UNIDIRECTIONALLY, AND THEN (WHILE THE COMPOUND IS WITHIN ITS SMECTIC C PHASE TEMPERATURE RANGE) VIEWED IN LIGHT THAT IS POLARIZED PLANARLY OR CIRCULARLY. THE TEMPERATURE INDICATION IS SENSITIVE TO CHANGES IN TEMPERATURE THAT ARE QUITE SMALL, AND THE RESPONSE PATTERN IS EXCEPTIONALLY RAPID (BEING ON THE ORDER OF A MILLISECOND AT THE GREATEST), SINCE NO MOLECULAR REARRANGEMENT IS INVOLVED.

United States Patent Int. Cl. G02f 1/16 U.S. Cl. 252-408 Claims ABSTRACTOF THE DISCLOSURE Temperatures may sometimes be quickly and convenientlysensed and/ or mapped with the use of a variable-tilt compound of theSmectic C mesomorphic phase. Disclosed are several organic compoundsthat are capable, within certain temperature ranges, of exhibiting sucha phase, such as terephthal-bis (4-n-butylaniline) and other compoundsresulting from the reaction of terephthaldehyde with amines or amides ofthe formula:

where A is --NH or -CONH and R is a saturated alkyl or alkoxy groupcontaining 1-12 carbon atoms. The compound is made, applied to a surfacethat has been rubbed unidirectionally, and then (while the compound iswithin its Smectic C phase temperature range) viewed in light that ispolarized planarly or circularly. The temperature indication issensitive to changes in temperature that are quite small, and theresponse pattern is exceptionally rapid (being on the order of amillisecond at the greatest), since no molecular rearrangement isinvolved.

BACKGROUND OF THE INVENTION (1) Field of the invention This inventionrelates to novel organic chemical compositions that exhibit avariable-tilt Smectic C phase and to a method of sensing or mappingtemperatures of an object with the use of such compositions.

(2) Description of the prior art SUMMARY OF THE INVENTION Temperaturesmay sometimes be quickly and conveniently sensed and/ or mapped with theuse of a variabletilt compound of the Smectic C mesomorphic phase.Disclosed are several organic compounds that are capable, within certaintemperature ranges, of exhibiting such a phase, such asterephtha1-bis-(4-n-butyl aniline) and other compounds resulting fromthe reaction of terephthaldehyde with amines or amides of the formula:

Q where A is -NH or CONH and R is a saturated alkyl or alkoxy groupcontaining 1-12 carbon atoms. The compound is made, applied to a surfacethat has been rubbed unidirectionally, and then (While the compound iswithin 3,723,346 Patented Mar. 27, 1973 its Smectic C phase temperaturerange) viewed in light that is polarized planarly or circularly. Thetemperature indication is sensitive to changes in temperature that arequite small, and the response pattern is exceptionally rapid (being onthe order of a millisecond at the greatest), since no molecularrearrangement is involved.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been unknown, prior tothe instant invention, that there are compounds of the Smectic C phasethat exhibit a temperature-dependent tilt angle. These com-' pounds ofthe Smectic C phase may be used for the measurement of temperature, atleast within the temperature domain wherein the compound involvedexhibits the Smectic C phase. A prerequisite to the practice of theinvention is that the molecules that are within the Smectic C phase areoriented in such a way that the layers are all parallel to the surfacesthat contain the liquid-crystal material. This parallelism may beaccomplished in various ways, such as by rubbing the containing surfaceswith a cotton cloth, or with paper or other material that has anorienting influence on the surface with which the liquidcrystal materialis in contact. Liquid-crystal material is then placed between the tworubbed surfaces in such a way that the rubbed directions of the surfacesare parallel. In some instances, a desired parallel orientation of thelayers may be achieved by containing the liquid-crystal material betweentwo surfaces, only one of which has been so unidirectionally rubbed.There is thus obtained a liquid-crystal unit, and when circularlypolarized light is caused to become incident upon the stratum ofliquidcrystal material, the liquid-crystal material gives a uniformretardation thereacross, provided that the temperature is constantacross the sample. If the temperature throughout the domain of thestratum of temperaturedependent variable-tilt Smectic C liquid-crystalmaterial is not uniform, the liquid-crystal material then does not havea tilt angle that is uniform, but rather has a tilt angle that varies inaccordance with the temperature in various portions of said domain, andthere is thus obtained an optical retardation that varies from onelocation to another within the sample, with the value of said opticalretardation depending upon the tilt angle at any particular locationwithin thesample. Thus, if the sample is observed in transmission of acircular polarizer having a sense opposite to that of the incidentlight, there is observed an interference color that is characteristic ofthe temperature of the liquid-crystal material at a particular pointwithin the above-mentioned domain.

One example of a material that exhibits the properties that arediscussed above is terephthal-bis-(4-n-butyl) aniline, which has avariable-tilt Smectic C phase in the temperature range of 172.5 C. to144.1 C. In that temperature range, the tilt angle changes from 0 degreeto 26 degrees. The rate of change of the tilt angle is not linear overthe temperature range. In the temperature range of 172.5 C. to 166 C.,the tilt angle changes rather rapidly at a rate exceeding 3 degrees perdegree centigrade, and from 166 C. to 144.1 C., the rate of change is onthe order of 0.5 degree per degree of centigrade. This change of tiltangle is reversible with temperature, so long as the liquid-crystalmaterial is not allowed to crystallize into a dilferent phase.

Terephthal-bis-(4-n-butyl)aniline may be prepared as follows: One mol ofterephthaldehyde in 8 mols of ethanol is mixed with 2 mols butylaniline. The mixture is refluxed for 16 hours. Terephthal-bis(4-n-butyl)aniline is obtained in percent yield, after recrystallizationfrom ethanol.

We have found, moreover, that the above-mentioned tilt angle isrepeatable or reproducible with temperature variations within a range ofor 0.2 degree centigrade. The reproducibility may be better than that,since the equipment used to determine such reproducibility was itselfonly accurate to within or 0.2 degree centigrade. Those skilled in theart will appreciate that liquidcrystal materials of the variable-tiltSmectic C phase comprise exceedingly good temperature indicators,because very small changes in optical retardation may easily be observedwith the use of suitably (planarly or circularly) polarized light.

There are other liquid-crystal materials or compounds that haveexhibited properties that indicate that they possess a variable-tiltSmetic C phase. Such compounds areterephthal-bis-(4-n-butylaminobenzoate); terephthal bis-(4-n-hexyloxyaniline); and terephthal-bis-(4-n-octyloxyaniline). TheirSmectic C temperature ranges are, respectively; 91 C. to 134 C.; 175 C.to 230 C.; and l73.7 C. to 231.5 C.

It may thus be stated that the compounds that exhibit the variable-tiltSmectic C phase appear for the most part to be products of the reactionof one mole of terephthaldehyde with 2 moles of an organic amine oramide of the class having the structure:

where A is an amine (--NH or an amide (-COHN group and R is a saturatedand preferably straight-tail aliphatic alkyl or alkoxy group containing1 to 12 carbon atoms. As will be apparent, the group R is located parato the reactive group A by means of which it is joined to theterephthaldehyde to form the compound in question.

There is thus obtained a compound of the structural formula:

where X is a radical selected from the group consisting of =N and Inaccordance with the further embodiment of the present invention, aSmectic C variable-tilt liquid-crystal material is caused to be orientedas previously described upon a metallic reflecting surface with theliquid-crystal material then being observed in reflected and circularlypolarized light. In this instance, the circularly polarized light thatis transmitted through the liquid-crystal material undergoes a phasechange upon reflection so that it is converted into circularly polarizedlight of a sense opposite to its original sense. Thus, when thereflected light is observed through the original circular polarizer,there is obtained a result that is equivalent to the viewing of a samplein transmission that is positioned between two polarizers of oppositesense. Again, as with the viewing of a sample of the liquid-crystalmaterial in question through which light has transmitted, there is anoptical retardation that depends upon the tilt angle, and thus withinthe temperature region involved, upon the temperature. If thetemperature is constant throughout the domain of the stratum ofliquid-crystal material, the interference color of the sample, viewed inaccordance with this embodiment of the invention, is uniformly the same,but, if there is a variation in temperature from one location to anotherthroughout the stratum comprising the domain, there is a correspondingvariation of the interference colors observed.

The kind of configuration discussed above is especially applicable tothermal imaging. A thin metal film is used as one of the containingsurfaces of the liquid-crystal material and thermal radiation is focusedupon such a film. The resulting variation in temperature from one pointto another throughout the stratum yields a visible and colored image ofthe thermal-radiation pattern. There is thus contained a thermal-imagingdevice that corresponds, in a sense, to that disclosed by Fergason inUS. Pat. No. 3,114,836, except that the devices made with the use of thevariable-tilt Smectic C liquid-crystal material of the present inventionhas the advantage of responding substantially more rapidly than thecholesteric-phase liquid-crystal material described by Fergason in theabove-mentioned patent, since no molecular rearrangement is involved.

Yet another embodiment of the instant invention comprises theobservation of the liquid-crystal material by means of transmittedplane-polarized light. In order that plane-polarized light can be usedunambiguously, it is essential that the layers of the liquid-crystalmaterial be parallel to the surfaces, and also that the long axes of themolecules be uniformly oriented in one direction. Uniform orientation ofthe long axes of the molecules is usually more difficult to obtain overlarge areas than is parallelism of the layers to the surface. If such anoriented variable-tilt Smectic C liquid-crystal material is placedbetween crossed or parallel polarizers, uniform interference color isobserved if the temperature across the sample is constant, but, again,if there is a. variation from one point to another in temperaturethroughout the domain of the stratum of variable-tilt Smectic Cliquid-crystal material, there is a corresponding variation from pointto point in the interference color that is observed. When linearpolarizers are used, the interference colors that are observed dependupon the orientation of the direction of the long axes of the moleculeswith respect to the privileged directions of the polarizers.

It is considered that the synthesis and the utilization of theorganic-chemical compounds involved lies well within the skill of theart, in view of the disclosure contained hereinabove, and it is furthersubmitted that it will be apparent to those skilled in the art how toconstruct a liquid-crystal unit wherein a suitable One of such compoundsis brought into the form of a stratum of a suitably orientedvariable-tilt Smectic C liquid-crystal material. Such stratum may be 0.1mil to 20 mils in thickness, being produced by applying the organiccompound itself or a suitable solution thereof in a volatile organicsolvent to the surface with which it is to be in contact.

It will also be apparent to those skilled in the art that the keeping ofthe liquid-crystal material within the desired temperature range may insome instances be conveniently accomplished by providing one of thestrata with which it is in contact with a suitable coating oftransparent electroconductive material such as indium oxide doped withtin oxide. Suitable bus bars and leads are provided, and by applying anappropriate amount of electrical power, there is generated enough heatto maintain the variable-tilt Smectic C liquid-crystal material within adesired temperature range. Those skilled in the art will perceive how,by shielding a portion of the domain from incident thermal radiation andusing it as a control viewed in monochromatic light, it is possible tocontrol G -aQ-WQ where X is selected from the group consisting of =N-and and R is selected from the group consisting of the aliphatic alkyland alkoxy radicals that contain 1 to 12 carbon atoms.

2. A composition as defined in claim 1, characterized in that thecomposition is within a temperature range such that said compoundexhibits a temperature-dependent variable-tilt Smectic C phase.

3. A composition as defined in claim 1, characterized in that saidcompound is terephthal-bis-(4-n-butylaniline).

4. A composition as defined in claim 3, characterized in that thecomposition is within a temperature range such that said compoundexhibits a temperature-dependent variable-tilt Smectic C phase.

5. A composition as defined in claim 1, characterized in that saidcompound is terephthal-bis-(4-n-butylaminobenzoate).

6. A composition as defined in claim 5, characterized in that thecomposition is within a temperature range such that said compoundexhibits a temperature-dependent variable-tilt Smectic C phase.

7. A composition as defined in claim 1, characterized in that saidcomposition is terephthal-bis-(4-n-hexyloxyaniline).

8. A composition as defined in claim 7, characterized in that thecomposition is within a temperature range such that said compoundexhibits a temperature-dependent variable-tilt Smectic C phase.

9. A composition as defined in claim 1, characterized in that saidcompound is terephthal-bis-(4-n-octyloxyaniline).

10. A composition as defined in claim 9, characterized in that thecomposition is within a temperature range such that said compoundexhibits a temperature-dependent variable-tilt Smectic C phase.

References Cited UNITED STATES PATENTS 3,655,270 4/1972 Creagh 252408ROBERT F. BURNETT, Primary Examiner M. E. McCAMISH, Assistant ExaminerUS. Cl. X.R.

23--230 LC; 16l--1, 5; 260-599; 350P

